Inter-satellite links are used in connection with communications satellite systems, which are moving in low earth orbits (LEO) and relate to so-called global communications networks, in order to connect the individual satellites of the system in the manner of a "network in the sky". Many satellite systems have already been planned or are even under construction. Satellites, which permit real time transmission of data, sound and video and are known as "big LEOs", are particularly important for the present invention. Examples of this are the so-called IRIDIUM and CELESTRI systems, originally introduced by Motorola, the TELEDISC system of Microsoft and McCaw Corp. It is expected that these systems will be functional around the year 2005.
The use of inter-satellite links (ISL), besides the customary earth links (downlinks and uplinks, i.e. from the satellite to the ground or vice versa), distinguishes the above mentioned LEO systems from others, such as GLOBALSTAR (by Globalstar Telecomms. Ltd.).
The main purpose of the global real time satellite communications networks lies in assuring a variable time-dependent transmission capacity to any location in the entire world when needed. Such a dynamic communications network requires real time management. In addition, a universal and easy access to the network is demanded when a wireless access is offered such as is the case, for example, with IRIDIUM. The determination of the exact orbital positions of all satellites is performed by the ground station.
Customarily the maintenance and control of an orbit of a satellite is controlled from the ground. The required orbital adjustment is calculated by the ground station, and a number of commands is generated and uplinked to the satellite, which are then downlinked again for checking. Finally, the ground station transmits a command for starting the control commands and the satellite performs them by using its own cycle generated on board. Such a sequence protects the satellite against transmission errors, since often it is out of sight of the ground station.
Recently autonomous navigation systems have aided in making an autonomous maintenance of the orbit possible, efficient and dependable. These systems can be aided by GPS, or can operate completely independently (MANS by Microcosm). In connection with LEO communications networks, the economical aspect of maintaining the orbit and the constellation are of decisive importance, since a large number of satellites--up to several hundred--must be controlled simultaneously.
The so-called checked autonomy, wherein the orbital maneuvers are initially calculated on board of each satellite, but are only executed after a check by the ground station, is a mechanism for reducing the risks and for utilizing the autonomy in maintaining the orbit.
The greater portion of the LEO satellite networks mentioned above consists of several orbital planes at the same height all around the earth. Generally all orbits of an LEO network have the same inclination in respect to the equator. The same number of satellites is distributed at the same distances from each other in each orbital plane. Such constellations are called "Walker" orbits. The total number of satellites, followed by the number of orbital planes and the inclination of the orbital plane, are characteristics, which permit a differentiation between all existing LEO network concepts.
There are sub-groups within a satellite network, which each have a predetermined number--between four and eight--satellites connected with each other for the mentioned communications purposes. Viewed from a satellite in the center of such a sub- group, two types of inter-satellite links can be distinguished: "inter-satellite links within the plane", i.e. links with satellites, which are ahead or behind, but in the same plane, and "inter-satellite links between the planes", i.e. links with satellites located in an adjacent plane or in a plane adjacent to the latter. An additional characteristic of each satellite network is expressed by the so-called "phase between the planes", the angular displacement, which constantly results during the course, between a central satellite of a sub-group and an adjacent satellite in the directly adjacent orbital plane.
An important problem, which results normally in each real time LEO satellite network operating with a fixed "phase between the planes", is to assure that the satellite of the same sub-group always remain in the same relation toward each other. This requirement is the result of the need to overlap the covered ground zones.
The employment of navigation methods performed or supported from the ground for monitoring the large number of satellites in LEO networks represents a relevant cost factor in maintaining the orbits during the working phase of an established satellite network.